1. Field of the Disclosure
The present disclosure relates to compensating for variations in air density in a pressure support device.
2. Description of the Related Art
It is known to treat a medical disorder or to diagnose, treat, or monitor the condition of a subject using medical equipment. For example, subjects suffering from a pulmonary or respiratory disorder, such as obstructive sleep apnea (OSA), are often treated with a pressure support device. One example of such a pressure support device is a continuous positive airway pressure (CPAP) device. A CPAP device delivers a flow of fluid to the airway of the subject throughout the subject's breathing cycle in order to “splint” the airway, thereby preventing its collapse during sleep.
Another example of a pressure support device provides a bi-level positive pressure therapy, in which the pressure of fluid delivered to the subject's airway varies or is synchronized with the subject's breathing cycle to maximize the medical effect and/or comfort to the subject. This type of device may be known as a bi-level positive airway pressure (BiPAP) device. With some BiPAP devices, a lower pressure is delivered to the subject during the subject's expiratory phase than during the inspiratory phase. It is also known to provide an auto-titration positive pressure therapy in which the pressure provided to the subject changes based on detected conditions of the subject. Such detected conditions may include whether the subject is snoring or experiencing an apnea, hypopnea, or upper airway resistance.
Once a subject is diagnosed with a breathing disorder, he or she is typically prescribed a pressure support therapy, i.e., a mode of pressure support (e.g., continuous, bi-level, or auto-titration), and given a prescribed pressure support level. The pressure support therapy (mode of pressure support and pressure settings) is typically prescribed by a physician after the subject undergoes a sleep study at a sleep lab.
Pressure support devices range from very simple to highly complex with regard to the approaches used to measure and control the pressure level of the air that is delivered to subjects. The simplest pressure support devices do not utilize any pressure sensors at all. Rather, they rely on a known relationship between the base motor speed and the output pressure for their pressure control, and they generally control the motor speed to that same level as long as therapy is delivered. However, it is well known that for a given motor speed, the output pressure of the blower will vary substantially with the density of the air (which is closely related to the air temperature and the absolute pressure (barometric pressure) of the air through the “ideal gas law”). Therefore, it is appropriate for these simplest of the pressure support devices to establish the base motor speed setting based on the local air density. Some existing pressure support devices include “manual altitude compensation” in which the base speed setting is adjusted based on a coarse setting for the local altitude, which is inputted to the pressure support device by a user. The altitude setting may be as coarse as having only three settings. For example, the altitude settings may include a setting of “1” for altitudes of 0-2500 ft above sea level, “2” for 2500-5000 ft, and “3” for 5000-7500 ft.
In contrast, more complex pressure support devices typically utilize a differential pressure sensor to directly measure the pressure support device's output pressure relative to the local ambient atmospheric pressure, and then control the blower speed to drive that pressure measurement to the proper (i.e., prescribed) level. In this scheme, the motor speed is generally and continuously driven to whatever RPM is necessary to generate the proper pressure for the patient. In these pressure support devices, the altitude compensation is fully automatic and does not require any intervention from the user. Further, the pressure control algorithm used in this approach does not require knowledge of the motor speed or of any of the local ambient variables of altitude, barometric air pressure, or air temperature in order to operate.
Because of the significant differences in how the simplest pressure support devices control pressure versus how the more complex pressure support devices control pressure, there are consequential differences in the accuracy of the pressure delivered to the subject, and there are significant cost differences in terms of components required by the respective approaches. In general, the more complex pressure support devices are far more accurate than their simpler counterparts (especially after taking into account output pressure variation that might occur due to ambient air pressure, temperature, and/or humidity deviations that may be encountered in the operating environment). The more complex pressure support devices are also generally more costly than their simpler counterparts due to presence of the differential pressure sensor and its associated signal paths and added complexities in the pressure support device's air path.